1. Field of the Invention
The present invention relates to a loudspeaker system and more particularly to a device for effectively enlarging the volume of a speaker enclosure of the loudspeaker system in order to increase the compliance that a vibratable cone of the loudspeaker system sees.
2. Description of the Prior Art
The fundamental physics of the pressure variations in a sound wave are discussed in Mechanics, Heat and Sound by Francis Weston Sears wherein on page 498 he sets out the following equation: EQU v.sup.2 = (.gamma.) (p/.rho. ),
where v is the speed of sound in a gas, (.gamma.) is the ratio, C.sub.p /C.sub.v of the specific heat of the gas at a constant pressure, C.sub.p, to the specific heat of the gas at a constant volume, C.sub.v, .sub.p is the pressure of the gas, and (.rho.) is the density of the gas.
In Applied Acoustics, Harry F. Olsen and Frank Massa discuss a back enclosed cone of a loudspeaker system on pages 197 and 198, the following is an excerpt from that discussion:
"In general cone speakers are used with both sides of the cone open so that radiation into the air takes place from both sides. For certain uses, as for example a standard source of sound for microphone calibration, reverberation measurement, it is desirable to enclose the speaker mechanism in a box and thus confine the radiation from the cone. The important factor in this system is the stiffness introduced by the box. The net result of this added stiffness is an attenuation of the low-frequency response. A specific example will illustrate the important factors in this system."
In the specific example, the velocity of the cone is given by the equation: ##EQU1## where f.sub.M = Bli = B(flux density in the air gap) 1 (length of wire in the voice coil) i (current in the voice coil), r.sub.M = radiation resistance, m = mass of the cone, voice coil and air load, C.sub.M = C.sub.M1 + C.sub.M2 = C.sub.M1 (compliance of center and suspension system of the cone) + C.sub.M2 (compliance of the box enclosing the back of the cone), C.sub.M2 = V (volume of the box)/ A.sup.2 (the square of the area of the cone) (.rho.) (the density of air) c.sup.2 (the square of the velocity of sound in air). Another excerpt from Applied Acoustics follows:
"From a consideration of the equation it will be seen that above the resonant frequency the velocity of the system is inversely proportional to the frequency; therefore, since r.sub.M is proportional to the square of the frequency the power output will be independent of the frequency. Below the resonant frequency the velocity is limited by the compliances C.sub.M1 and C.sub.M2 and the velocity of the cone is practically proportional to the frequency, which means that the response is rapidly attenuated with decreasing frequency. Therefore, the low-frequency response limit will be determined by the resonance frequency of the system. If C.sub.M2 is large compared to C.sub.M1, then the compliance of the box will not materially affect the response and the action will be practically the same as that with both sides open to the air. If the resulting volume when the condition is satisfied is too large and cumbersome, then the system must be altered. Since the mass of the cone is practically proportional to the area and its interaction with the compliance C.sub.M2 is inversely proportional to the square of the area, we can reduce the resonance frequency by reducing the area of the cone." A reduced area will require an increased excursion for equal power resulting in increased distortion.
U.S. Pat. No. 3,905,448, entitled Loudspeaker, issued to Hirotake Kawakami, Toshio Sasabe, Toshio Hirosawa, Nobuyuki Arakawa, Kozo Kokubu, Kazumasa Abe and Toshiko Harashino on Sept. 16, 1975 teaches a loudspeaker of a general type having a vibratable cone diaphragm.
U.S. Pat. No. 2,797,766, entitled Loud Speaker, issued to Herbert W. Sullivan on July 2, 1957 an air tight enclosure containing an acoustic diaphragm is provided with a membrane substantially permeable to mechanical vibrations, but substantially impermeable to the gaseous medium on either side of the membrane. The acoustic diaphragm vibrates in a gaseous medium which is heavier than air and in which sound travels at a slower speed than in air. The characteristic impedance of the diaphragm in the gaseous medium and the acoustical capacitance is lower than when compared to those prevailing in air. The difficulty with this air-tight enclosure is that the preferred speaker enclosure has a venting port so that there is a source of high velocity air against the back of the diaphragm of the loudspeaker.
U.S. Pat. No. 4,004,094, entitled Enclosure System For Sound Generators issued to James H. Ott on Jan. 18, 1977, teaches a device for use in an enclosure associated with an audio speaker which permits relatively large volume changes within the enclosure as a result of relatively small pressure changes so that relatively small enclosures can be effective as enclosures of larger volume. The device reduces the energy required from the speaker to change the volume of the interior of the enclosure. Pressure perturbations caused by the movement of the vibratorily driven membrane of the sound producing device cause alternate condensation and vaporization of the composition to minimize backpressure. This gas-liquid equilibrium is the key to the operation of this device. The patent teaches an improved sound production system which has a less than perfectly sealed enclosure with a flexibly walled container contained therein. The container has an expansible and contractible volume and contains a composition of matter having an equilibrium state between a gas phase and a liquid phase.
Loudspeakers presently in use are of the acoustical suspension design and require a large amount of power. There is a movement toward more efficient loudspeakers because individuals are demanding more volume with less distortion than typical low efficiency speakers can deliver with amplifiers of moderate size. The super-powered amplifiers in home music systems require several hundred watts of amplifier power in order to achieve faithful reproduction of modern recordings with low efficiency speakers. These speakers are unable to hold up under this deluge of amplifier power and they eventually break down. There is therefore a need for a high efficiency speaker that will require less power for lifelike reproduction of modern recordings.